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Fluid-Rock Interactions and the Sub-seafloor Biosphere

The upper oceanic crust comprises the largest aquifer on Earth. Fluid circulation within this aquifer influences the thermal state and composition of oceanic plates; interacts with hot, newly emplaced volcanic crust to form spectacular “black smoker” hydrothermal vents with their unique biological communities; concentrates massive reservoirs of methane and methane hydrates along continental margins; and hosts a vast, largely unexplored sub-seafloor microbial biosphere. There is increasing evidence that transient events—earthquakes, volcanic eruptions, massive slope failures—play a critical role in fluid-rock interact ions and sub-seafloor microbial activity. These transient events, which may last only hours or days, are very difficult to observe and sample using conventional ship-based studies. Understanding the linkages and feedback mechanisms among geological, chemical, and biological processes within these highly dynamic environments requires long-term, in situ observations, such as those provided by the OOI Cabled Array infrastructure.

Studies significant to understanding these systems include investigations of

  • The geological processes that form and age the oceanic lithosphere from birth to destruction.
  • The role of mid-ocean ridge volcanoes and flow in subduction zones in fostering diverse and productive biological communities above and below the seafloor.
  • The fluxes of heat, chemicals, and biomass across the seafloor and their effect on the overlying ocean.
  • The impact of perturbation events such as magmatic intrusion and earthquakes on geological, chemical and biological processes in the seafloor and within the overlying ocean.

Fluid circulation continues throughout the earth’s crust from spreading centers to the trench, influencing the thermal, mechanical, and chemical state of the subducting slab. Investigation of linkages of earthquakes and geothermal springs in continental systems shows that earthquakes impact fluid flux and the temperature of springs several hundreds of kilometers away from the epicenters.

Two major driving questions for the OOI include (1) how does plate-scale deformation mediate fluid flow, chemical and heat fluxes, and microbial productivity and (2) the spatial and temporal hydrologic connectivity of the oceanic crust and the impact that perturbation events have on plate-scale fluid transport and associated chemical and biological processes.

The major volcanic, magmatic and tectonic events that create the oceanic crust and modulate the fluxes across the seafloor and associated health of the biological communities are inherently episodic on decadal time scales and they are also short-lived. Transient events such as magmatic eruptions at mid-ocean ridges increase carbon dioxide output and venting volume by as much as a factor of 100, resulting in extensive microbial blooms. In margin environments, tectonic events release significant quantities of methane gas into the overlying sediments and hydrosphere, which may profoundly perturb microbial communities that thrive on sulfate and methane in these systems: Indeed, catastrophic release of methane hydrates is believed to lead to profound global changes in climate.

The OOI will allow scientists to capture these events by maintaining a long-term monitoring capability at a number of sites with high probability for tectonic and magmatic activity along major plate boundaries at the seafloor surface (and subseafloor through sealed boreholes) and have response capabilities to allow real-time capture of these events.

Related Science Questions

  • How does plate scale deformation mediate fluid flow, chemical and heat fluxes, and microbial productivity?
  • What are the temporal and spatial scales over which seismic activity impacts crustal hydrology?
  • How do the temperature, chemistry, and velocity of hydrothermal flow change temporally and spatially in subsurface, black smoker, diffuse, cold seep, and plume environments? How are these systems impacted by tectonic and magmatic events?
  • What is the composition and concentration of microbial material in subsurface, black smoker, diffuse, cold seep, and plume environments in time and space? How are these systems impacted by tectonic and magmatic events?

Gas Hydrates

A significant amount of methane near the surface of the Earth is locked into gas hydrates in the shallow sediments on continental margins. The hydrates may act as a capacitor in the carbon cycle by slowly storing methane that can be suddenly released into the ocean and atmosphere during seismic events or slope failure. Hydrate Ridge in the Cascadia accretionary complex is one of the best-studied gas hydrate deposits. Significant methane seeps hosting diverse biological assemblages and formation of gas-rich hydrate deposits near the seafloor have been documented. Studies of these deposits have provided a good understanding of how gas hydrate is distributed in marine sediments and of the processes that lead to heterogeneity in distribution. This site is a clear target for the observatory to define the temporal evolution of these dynamic systems, to determine the fluxes of methane from the seafloor into the ocean, and to understand the biogeochemical coupling associated with gas hydrate formation and destruction.

The real-time interactive capabilities of the OOI are critical to studying gas hydrate systems because many of the key processes may occur over short time scales and will require adaptive response and sampling capabilities that include fluid sampling, increases in data accumulation rates and imagery from cameras, and in situ manipulation of chemical sensors. As outlined in the Gas-Hydrate Observatories Workshop (2007), the high power and bandwidth capability of the Cabled Array component of the OOI is required to enable human-made perturbations to the system (e.g. fluid pumping; heating of the system to avoid hydrate formation during fluid sampling or to perturb the hydrates), operations that include downhole seismic and/or electromagnetic sources, multi-year deployments that are needed to capture the various time scales operating in this system, and the need for real-time intervention to capture infrequent events or otherwise change experiment parameters that cannot be made by passive monitoring.

Related Science Questions

  • How do tectonic, oceanographic and biologic processes modulate the flux of carbon into and out of submarine gas hydrate “capacitor”, and are there dynamic feedbacks between the gas hydrate methane reservoir and other benthic, oceanic and atmospheric processes?
  • What is the role of tectonic, tidal, and other forces in driving the flux of carbon into and out of the gas hydrate stability zone out of the sediment? How is this response influenced by geologic parameter (stratigraphy and structure)?
  • What is the significance of pressure change on hydrate stability and methane fluxes due to winter storms and pressure pulses, and bottom currents interacting with topography?
  • Can natural temperature fluctuations help us understand the effects of long-term temperature change on hydrate stability, or are perturbation experiments requirements to artificially raise the temperature?
  • What is the fate of hydrate/seep methane in the ocean and atmosphere? Does significant methane arrive to the atmosphere from hydrate sources?
  • Are there temporal variations in animal/microbial activity and composition that are affected by temporal variation in fluid flow, chemistry and flux?